The gain error, e.g., the difference between the actual transfer function and an ideal transfer function of a current feed-back instrumentation amplifier (CFIA) depends on the accuracy of matching between the CFIA's input transconductor and feedback transconductor. Some existing CFIAs try to address the gain error using an architecture in which each transconductor includes pre-amplifiers to create a transfer function proportional to the ratio between the degeneration resistors of the input and feedback transconductors, see FIG. 1. The advantage of this circuit architecture is that it provides a gain error that depends almost exclusively on matching of resistors which is superior to the matching of transistors. While the preamplifiers provide an overall low gain error, their noise adds to the total noise of the instrumentation amplifier, making this architecture less suitable for low noise designs. Another disadvantage is the presence of feedback loops around each pre-amplifier, which makes frequency compensation rather complex and difficult.
In another approach, the CFIA's transconductors are each based on a differential pair of transistors with degeneration. The noise is significantly lower than the solution that uses preamplifiers, and there will be no extra feedback loops. However, the gain error will also change: it will now be affected not only by the mismatch between the input and feedback degeneration resistors, but also by any mismatch between the degenerated differential paired transistors of each transconductor, and by any mismatch between the respective tail currents being fed into each transconductor. As noted above, resistors match much better than transistors do, so mismatches other than the mismatch between the degeneration resistors dominate the gain error.